Metered connection is a Windows 10 setting, not an Outlook setting, though Outlook recognizes when it's using an internet connection designated as Metered by Windows 10.
Make sure you're connected to your phone as your WiFi connection before proceeding
Settings, Network & Internet, Wi-Fi Pane, Select the connection that you're currently connected to and activate it (it acts as though it's a button). In the dialog that's now up for that connection, find the Metered Connection section and turn the toggle for Set as metered connection to OFF.
Once you've done this the connection is no longer metered and Outlook will not issue the warning.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore